The invention relates to a process for cooling polymerization-loop-slurry reactors in the preparation of polyolefins. Generally speaking, polymerization-loop-slurry reactors are used to house the circulation of a slurry comprising olefin, catalyst, polyolefin, and diluents. Loop-slurry reactors have a series of straight pipes connected by appropriate bent pipes, which form a continuous loop. Most loop-slurry reactors have four, six, or eight straight pipes segments, which are called legs. The loop-slurry reactor used according to the present invention may be any loop reactor known in the art to be used for slurry polymerizations. An example of such a loop-slurry reactor is described in U.S. Pat. No. 5,565,175, which is incorporated by reference in full.
The conversion of olefins to polyolefins in these reactors is an exothermic process. Accordingly, the heat of the reaction must be removed so that the temperature of the slurry in the loop-slurry reactor can be controlled. Each leg is surrounded by a jacket. The leg jacket is defined as an external jacket, sleeve, or pipe, through which a cooling fluid flows and absorbs heat emitted from the leg. In an embodiment the leg jacket is a second pipe that is concentric to and surrounding the reactor pipe. A cooling fluid, (typically water) that is at a lower temperature than the reactor contents, flows through the leg jacket and heat is transferred from the reactor contents to the cooling fluid. Suitable leg jackets may be obtained from Fabricom Company located in Belgium.
In a Conventional Process—described more fully below with reference to FIG. 1—a first cooling liquid, typically water, is pumped sequentially through each external jacket to absorb the heat of reaction and control the reactor temperature. As the first cooling liquid flows through each of the external jackets, heat is transferred from the reactor to the cooling liquid, and the temperature of the cooling liquid increases. The first cooling liquid is passed through a heat exchanger where the absorbed heat of the first cooling liquid is transferred to an external cooling liquid, typically water. The external cooling liquid is either discarded or preferably recycled into some other process. The external cooling liquid is preferably derived from a large source of readily available water such as a cooling tower or sea water. Accordingly, in some embodiments the temperature of the external cooling liquid is dependent upon the temperature of the environment. The first cooling liquid is re-circulated through the external jackets and the heat exchanger with the use of a pump.
In recent years, advances in catalysis and other process conditions have allowed for increased polymer production rates, which correspondingly increase the heat of reaction. Accordingly, there is a need to remove this increased amount of heat in order to maintain or improve productivity. One method for increasing the heat removal capability of the above-described Conventional Process is to reduce the temperature of the first cooling liquid that enters the external jackets. However, this method is limited, in practice, to the extent that the temperature of the cooling liquid cannot be reduced lower than the temperature of the external cooling liquid. And the temperature of the external cooling liquid is fixed because it is preferably dependent on the environment.
A second method for increasing the heat removal capability of the above-described Conventional Process is to increase the recirculation rate of the first cooling liquid. In an embodiment, between about 7 to about 10 percent more heat can be removed when the recirculation rate is doubled. Increasing the recirculation rate of the first cooling liquid requires either increasing the velocity of the cooling liquid in the space between the jackets and the reactor legs or increasing the size of the external jackets and the interconnecting pipes. However, because pressure drop is proportional to the square of velocity, doubling the velocity of the first cooling liquid will cause the pressure drop across the pump to increase by at least twice. Additionally, increasing the velocity of the first cooling liquid could cause long term erosion of the reactor nozzles and jackets. With respect to the possibility of redesigning the external jackets, this option is timely and expensive and is not practical for application to existing reactors.
U.S. Pat. No. 6,235,852 (“Hess”) discloses a process for cooling polymerization reactors in the preparation of polyolefins, the polymerization being carried out in a first reactor and in at least one further reactor, the further reactor or reactors being connected downstream of the first reactor and each being cooled by an internal cooling circuit in which a cooling medium circulates.
Accordingly, there is a need for a process that can increase the amount of heat removed from a single polymerization reactor, as well as a reactor system, while minimizing the increase in pressure drop, does not require extensive modifications, and will not erode the systems conduits or external jackets.